There are many oil field applications which require downhole pressure and differential pressure measurements. For applications such as permanent monitoring of completion systems and logging/measurement in producing zones, where the deployment of the sensor may be done via a ¼″ hydraulic control line, reliable and compact sensors which occupy minimum space are much desired.
There are also many applications where a differential pressure measurement is required. Typical examples include the determination of flow rate through flow control valves, down hole pump monitoring, Venturi based flow meters and density measurement based on manometer principle. In these applications, the measurement range varies from a few psi to a few thousand psi and for some applications accuracies better than +/−0.5 psi are required.
For permanent placement in a high pressure (above 1000 psi or 3000 psi or more) and possibly high temperature (above 70 or 90, or sometime even above 100 degrees Celsius) environment, it is important to provide sensors with a high reliability, measuring stability and a long lifetime. To date, there is no suitable differential pressure sensor available for such applications, at least in the field of monitoring of hydrocarbon reservoirs. Typically, two absolute pressure sensors are applied for such differential measurement. This approach has limited accuracy and increases the costs.
Silicon-based differential pressure sensors have been used in downhole production logging (PL) tools. These sensors, having their maximum measurement range limited to just a few psi, are unsuitable for high range applications such as monitoring the downhole flow valves and downhole pumps. As the demand for permanent downhole differential pressure measurement increases, so does the demand for a high quality and high range downhole differential pressure sensor.
Schlumberger has developed an absolute pressure sensor based on sapphire material. This is a very high performance sensor which has achieved an impressive track record during field applications. Typical performance data include excellent stability of 1 psi/year over a 10000 psi measurement range of 10000 psi, a resolution of 0.05 psi and accuracy to 0.01% of the full scale.
There are some disadvantages associated with the known sensor. For example in its usual configuration it cannot be used to measure differential pressure. And as the pressure to be measured is applied to the side where the electrical connections to the strain gauges are made, isolation is required between the actual sensor and the pressurized borehole fluid. This limits the possible reduction of the sensor package size.
It is therefore an object of the present invention to improve and enhance the capabilities of existing ruggedized pressure sensors.